crypto/internal/boring/bcache: make Cache type-safe using generics

crypto/internal/boring/bcache: make Cache type-safe using generics

Generics lets us write Cache[K, V] instead of using unsafe.Pointer,
which lets us remove all the uses of package unsafe around the
uses of the cache.

I tried to do Cache[*K, *V] instead of Cache[K, V] but that was not possible.

Change-Id: If3b54cf4c8d2a44879a5f343fd91ecff096537e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/423357
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>